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    NaN:not a number 
    当数学计算没有结果的时候,我们就会输出一个NaN
    <script>
        console.log("zhangdan"-"dingzhen");//NaN
    </script>

    isNaN():判断一个非数字
    is not a number 是不是  不是个数字
    isNaN(3) :false
    isNaN(a) :true
    <script>
        var n1=100;
        var n2="hi 帅哥";
        console.log(isNaN(n1));//false
        console.log(isNaN(n2));//true
    </script>
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